Electro-slag furnace for producing continuous ingot

ABSTRACT

An electro-slag furnace for producing a continuous metallic ingot comprises a housing having a bottom portion in which an electrode constituted of the forming ingot is shiftable. A further electrode is spaced above the ingot electrode and a cooling channel is provided to cool the slag in the furnace to form a slag crust on the wall of the channel. The depth of the bottom portion between the electrodes is determined as a function of the diameter of the channel such that the thickness of the crust layer on the channel wall relative to the length thereof between the electrodes is sufficient for preventing a shunting electric discharge between the channel wall and slag.

United States Patent [191 Kolisnyk Oct. 311, i973 1 1 ELECTRO-SLAGFURNACE FOR PRODUCING CONTINUOUS INGOT 211 Appl. No.: 153,294

Related US. Application Data [63] Continuation-in-part of Ser. No.692,452, Dec. 21,

1967, abandoned.

[52] US. C1. 164/252 [51] Int. Cl 322d 27/02 [58] Field 01828111211164/50, 52, 252

[56] References Cited UNITED STATES PATENTS 2,814,658 11/1957 Udy 13/33OTHER PUBLICATIONS Maksimovich, Avt. Svarka, 1961, No. 4, pp. 47-53.

(translation pages are 42-48).

Primary Examiner-J. Spencer Overholser Assistant Examiner-John E.Roethel Attorney-Eric l-l. Waters et al.

[57] ABSTRACT An electro-slag furnace for producing a continuousmetallic ingot comprises a housing having a bottom portion in which anelectrode constituted of the forming ingot is shiftable. A furtherelectrode is spaced above the ingot electrode and a cooling channel isprovided to cool the slag in the furnace to form a slag crust on thewall of the channel. The depth of the bottom portion between theelectrodes is determined as a function of the diameter of the channelsuch that the thickness of the crust layer on the channel wall relativeto the length thereof between the electrodes is sufficient forpreventing a shunting electric discharge between the channel wall andslag.

5 Claims, 1 Drawing Figure ELECTRO-SLAG FURNACE FOR PRODUCING CONTINUOUSINGOT CROSS-RELATED APPLICATIONS This is a continuation-in-partapplication of my previously co-pending parent application Ser. No.692,452 filed on Dec. 21, i967 and now abandoned.

. SUMMARY OF THE INVENTION The present invention relates to metallurgy,and, more particularly, to 'electro-slag furnaces for producing acontinuous metallic ingot.

Known in the art is an electro-slag furnace for producing a continuousmetallic ingot, the furnace comprising a cooled hollow housing filledwith molten slag and communicating with a mold through an openingprovided in the bottom of the housing, the mold being electricallyinsulated from the latter. In the zone of said opening, the bottom ofthe housing is fashioned as a copper pipe line with cooled hollow wallsextending downwardly with respect to said housing, said pipe line beinginstalled in said mold. The depth of the opening exceeds its diameter byapproximately six times. Metal is cast in the cooled housing with theaid of consumable melting electrodes which are essentially hollowcylinders filled with metallic particles and manufactured from a bandwith the aid of rolls. The metal forms a pool on the bottom of thehousing, flows into the mold through said pipe line and forms a pool onthe ingot. The housing of the furnace and the molds are connected to anelectric current source. An electric discharge takes place between theend portion of the cooled pipe line disposed in the mold and the ingotto pass through the molten slag. This discharge heats the metal of thepool and the slag in the mold (cf. U.S. Pat. No. 2,445,670; Cl. 22-572,1948).

When using the conventional furnace of the aforedescribed type therearise difficulties entailed by the necessity to provide equipment formanufacturing sufficiently intricate electrodes and to consume themetallic band. Moreover, to heat the metal and the slag, the mold canemploy a dc source whose poles are connected in a strictly definitemanner. In this respect, the negative pole must be connected to thehousing and the positive pole must be connected to the mold.

This manner of connection of the poles of the dc. source, however,inevitably results in electrolysis of the molten slag, which lessens thequality of the metal of the ingot and changes the composition of theslag in the mold.

Furthermore, a portion of the current supplied by the source flowsbetween said pools, and this portion is absolutely insufficient forforming a pool, thus necessitating employment of a plurality ofelectrodes. Such arrangements are shown in U.S. Pat. Nos. 3,507,968 and3,51 1,303.

It is an object of the present invention to obviate the housing with athickness or depth which is sufficiently small to preclude an electricdischarge between the bottom and the molten slag. The electrodes arepositioned such that one is mounted on the bottom of the housing and theother, which is essentially the ingot, is connected to an electriccurrent source whose power is sufficient for providing for the formationof a pool of molten metal on the electrodes. The spaces of the cooledhousing and the mold are in communication with a pump providing forflowing of coolant at a rate sufficient for maintaining the skin of thecooled slag on the walls of the opening and the mold.

This embodiment of the furnace allows dispensing with a supply of metalin the form of a consumable electrode. The metal can be supplied inpractically any form, including a compound of metal with a reducingagent. It is also possible to use alternating current.

In the preferred embodiment of the furnace, accor ding to the presentinvention, it is expedient to make the thickness of the bottom portionof the housing, determining the depth of the opening through which thefurnace housing communicates with the mold, equal to approximately onetenth of the lateral dimension or diameter of the opening.

With the bottom portion of the housing embodied as described above, thethickness of the skin or crust of the slag formed on the walls of theopening is sufficient to preclude an electric discharge between saidbottom portion and the molten slag.

A substantial advantage of the present invention is that the metal canbe deposited or charged into the furnace in the form of separateparticles of any shape, that are not connected to the electric currentsource.

The present invention therefore contemplates to an electro-slag furnacecomprising a chamber containing molten slag, means for introducing metalparticles into the molten slag, a first electrode fixed in the chamber,a second displaceable electrode spaced from the first electrode andconstituted by solidified metal derived from said metal particles, meansfor applying electrical energy to said electrodes, and cooling means forcooling the chamber between said electrodes for forming a slag crustwhich prevents short circuit between the'second electrode and thechamber.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of the drawing is adiagrammatic, vertical sectional view of the furnace according to theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT:

The electro-slag furnace comprises a cooled hollow housing I having ametallic electrode 2 mounted therein. The electrode '2 is mounted on acooled bottom portion 3 of the housing 1 filled up with molten slag 4.The housing 1 communicates through an opening or channel provided in thebottom portion 3 thereof with a cooled mold 5 accommodating an ingot 6serving as a second metallic electrode. The electrodes 2 and 6 areconnected to electric current source 7. The power of the source 7 mustbe sufficient to melt the electrodes 2 and 6 and to form on theirsurfaces pools of molten metal. It is desirable to use an ac source asthe electric current source 7. The bottom portion 3 of the housing 1 andthe mold 5 are each electrically insulated from each other by a packing8 constituted of asbestos. Provided between the bottom portion 3 of thehousing 1 and the metallic electrode 2 is also a layer of an electroandthermo-insulating material, i.e., asbestos packing 9. The housing 1 isprovided with slag hole 10. The walls of the housing 1, bottom portion 3and mold 5 are cooled by circulating medium 11. These walls can be madefrom copper. The spaces of said walls are in communication with a pump(not shown in the drawing) providing for the flow of cooling medium 11at a rate sufficient to preserve or maintain skin 12 of the cooled slagas a crust formation on the bottom portion 3 and mold 5. As compared tothe molten slag 4, the skin or crust 12 has a lower electricalconductivity.

Prior to the electro-slag furnace operation, the pump is started so asto fill the furnace with the molten slag 4, and the electric currentsource 7 is then switched on. Lines 13 conventionally show the paths offlow of the electric current.

In order to operate the furnace, it is necessary to make the thicknessor depth of the bottom portion 3 sufficiently small to prevent ashunting electric discharge between the bottom portion 3 and the moltenslag 4. A drop in voltage in the column of the molten slag 4 inside theopening in the bottom portion 3 will not result in or maintain saidelectric discharge which can destroy the slag crust layer on the wall ofthe bottom portion 3 and/or prevent the formation of a pool of moltenmetal on the electrode 2.

It is desirable to make the thickness or depth H of said bottom portion3 equal to one tenth of the lateral dimension or diameter D of thechannel or opening in the bottom portion 3, which is confirmed bycasting a metal ingot in molten calcium fluoride.

An ingot of metal is cast by supplying a metal charge 14 onto thesurface of the molten slag 4. This charge melts on the surface of themolten slag 4 upon contacting the latter, and there results theformation of metallic drops 15 which settle down in a pool of liquidmetal on the ingot 6. When using a charge 14 composed of large pieces,such as granules or scrap, the latter are supplied into the zone of themolten slag 4 above the electrode 2 to settle down'onto the electrode 2to be melted thereon, and thus flow into a pool on the ingot 6. Bygradually supplying the charge 14 and maintaining a continuous film, theingot becomes larger by absorbing the solidifying molten metal of thepool.

The ingot is gradually moved down from the mold, which is usually thecase during continuous casting.

The metal charge for producing ingots may be supplied in particulateform or as fusible rods or wire, bars such as conventional electro-slagremelts, or as a mixture of a chemical compound of a metal and arecovering agent, or as a liquid or as a combination of theaforementioned forms.

In sum, the solidification and cooling of the slag which is accompaniedby a considerable decrease in electric conductivity is utilized toprovide an electrical insulation in the form of the crust of slagsolidified on the cooled metal surfaces in contact with the molten slag.

Note should be made to the effect that the electrical conductivity ofthe crust of solidified slag depends upon the composition of slag andtemperature of crust and, consequently, upon the thickness and type ofmaterial of the wall being cooled on which said crust has solidified, aswell as upon the intensity with which said wall is cooled and upon thetemperature of the molten slag. Fluoride slags (such as CaF and 5% CaO,or 65% CaF 30% A1 0 and 5% CaO, and the like) feature a high electricalconductivity in the molten state, forming during solidifcation a crusthaving a lower electric strength as compared with oxide slags (such as55% A1 0 and 45% CaO, or 40% M 0 25% CaO, 17% MgO and 18% CaF Therefore,by making use of cooled metal elements coated with a crust of solidifiedslag, there can be insulated from each other the electrodes, and thepools of liquid metal, placed in the molten slag.

In order to make the current between the liquid metal baths sufficientfor melting the metal in said baths, the following conditions must bemet:

a. to provide for sufficient cooling to insure the formation thereonoflow-electroconductive crust of solidified slag serving to preclude anelectrical discharge be tween the electrodes and the housing 1, thebottom 3, and mold 5, and

b. to considerably reduce the length of the bottom 3 so that, during thepassage of electric current between the baths, across the molten slagcolumn inside the bottom 3 there should occur no excessive voltage dropthat could bring about an electrical breakdown of the slag crustinsulation and cause a shunting discharge between the bottom 3 and themolten slag.

By reducing the ratio between the height of the molten slag column inthe bottom 3 and the diameter or cross-section of said column, thevoltage drop across said column can be reduced, other things beingequal, while providing discharge between the liquid metal baths orpools.

The requirements placed upon the proportions of the cooled metal elementof an electroslag furnace containing molten slag through which electriccurrent passes under conditions of electroslag remelting are as follows:

U HI/aS and U 4HI/ourrD wherein:

U is the voltage drop across the slag column;

a is the specific electric conductivity of the molten slag at a presettemperature;

H is the height of the slag column;

I is the melting current;

S is the cross-section area of the slag column; and

D is the diameter of the slag column.

In a first approximation, the current strength during the electroslagprocess is determined by the specific electric conductivity of themolten slag and can be proportional to the diameter of the molten slagcolumn or to the square root of the cross-sectional area of the slagcolumn or to the perimeter of the cross-section of said column. Based onthe conditions of electro-slag remelting and on specific electricconductivity of molten slags at the temperatures of the electroslagprocess, and neglecting the small thickness of the slag crust ascompared to the dimensions of the electroslag furnace, approximateresults are presented in the appended Table.

Based on indirect data, it is concluded that the voltage of the electricbreakdown of the slag crust under conditions of electroslag remeltingusing water-cooled copper elements is approximately equal to 15-100volts. It follows from the data on the Table that, when using cooledelements with a molten slag column featuring an H/D ratio exceeding l-2,electrical breakdown of even a double layer of slag crust is likely,which can bring about the appearance in the place of this breakdown ofan intensive discharge between the cooled wall and the molten slag.

In order to preclude such a discharge, a readily attainable andadequately reliable solution for a cylindrical column of molten slag isan H/D ratio of about 0.1. At such H/D ratio, the voltage drop acrossthe molten slag column is less than the voltage of the electricalbreakdown of the slag crust. A further decrease of said ratio makes foran increased reliability of the furnace operation. The above method ofcalculation can likewise be used in the case of cross-sectional shapesof the molten slag column which are other than cylindrical;

TABLE Voltage drop across molten slag column under conditions ofelectroslag remelting Voltage What is claimed is:

1. An electro-slag furnace comprising a chamber containing molten slag,means for introducing a metal charge into the molten slag, a firstelectrode fixed in the chamber, a second displaceable electrode spacedfrom the first electrode and constituted by solidified metal derivedfrom said metal charge, means for applying electrical energy to saidelectrodes, cooling means for cooling the chamber between saidelectrodes for forming a slag crust which prevents short circuit betweenthe second electrode and the chamber, said chamber comprising a housingprovided with a bottom opening and a mold in extension of the opening,said second electrode being positioned in said mold, the electricalenergy applied to the electrodes forming pools of said metal charge atthe surfaces thereof, and means for maintaining the surface of thesecond electrode spaced from said bottom opening by a distancesubstantially less than the diameter of said opening.

2. A furnace as claimed in claim 1 wherein said housing has across-section which is greater than the size of said bottom opening.

3. A furnace as claimed in claim 1 comprising further cooling meansincluding a plurality of axially spaced coolant channels surroundingsaid housing bottom and said mold respectively.

4. An electro-slag furnace comprising a chamber containing molten slag,means for introducing a metal charge into the molten slag, a firstelectrode fixed in the chamber, a second displaceable electrode spacedfrom the first electrode and constituted by solidified metal derivedfrom said metal charge, means for applying electrical energy to saidelectrodes, cooling means for cooling the chamber between saidelectrodes for forming a slag crust which prevents short'circuit betweensaid molten slag and the chamber, said chamber comprising a housingprovided with a bottom opening and a mold below the opening in extensionthereof, said second electrode being positioned in said mold, theelectrical energy applied to the electrodes forming pools of said metalcharge at the surfaces thereof, the distance between the surface of saidsecond electrode and said bottom opening being no more than one-tenththe diameter of said opening.

5. An electro-slag furnace comprising a chamber containing molten slag,means for introducing a metal charge into the molten slag, a firstelectrode fixed in the chamber, a second displaceable electrode spacedfrom the first electrode and constituted by solidified metal derivedfrom said metal charge, means for applying electrical energy to saidelectrodes, cooling means for cooling the chamber between saidelectrodes for forming a slag crust which prevents short circuitsbetween said molten slag and the chamber, said chamber comprising ahousing provided with a bottom opening and a mold below the opening onextension thereof, said second electrode being positioned in said mold,the electrical energy applied to the electrodes forming pools of saidmetal charge at the surfaces thereof, the height of said housingdisposed between the first and the second electrodes being about tentimes smaller than the distance between the opposite sides of saidopening.

1. An electro-slag furnace comprising a chamber containing molten slag,means for introducing a metal charge into the molten slag, a firstelectrode fixed in the chamber, a second displaceable electrode spacedfrom the first electrode and constituted by solidified metal derivedfrom said metal charge, means for applying electrical energy to saidelectrodes, cooling means for cooling the chamber between saidelectrodes for forming a slag crust which prevents short circuit betweenthe second electrode and the chamber, said chamber comprising a housingprovided with a bottom opening and a mold in extension of the opening,said second electrode being positioned in said mold, the electricalenergy applied to the electrodes forming pools of said metal charge atthe surfaces thereof, and means for maintaining the surface of thesecond electrode spaced from said bottom opening by a distancesubstantially less than the diameter of said opening.
 2. A furnace asclaimed in claim 1 wherein said housing has a cross-section which isgreater than the size of said bottom opening.
 3. A furnace as claimed inclaim 1 comprising further cooling means including a plurality ofaxially spaced coolant channels surrounding said housing bottom and saidmold respectively.
 4. An electro-slag furnace comprising a chambercontaining molten slag, means for introducing a metal charge into themolten slag, a first electrode fixed in the chamber, a seconddisplaceable electrode spaced from the first electrode and constitutedby solidified metal derived from said metal charge, means for applyingelectrical energy to said electrodes, cooling means for cooling thechamber between said electrodes for forming a slag crust which preventsshort circuit between said molten slag and the chamber, said chambercomprising a housing provided with a bottom opening and a mold below theopening in extension thereof, said second electrode being positioned insaid mold, the electrical energy applied to the electrodes forming poolsof said metal charge at the surfaces thereof, the distance between thesurface of said second electrode and said bottom opening being no morethan one-tenth the diameter of said opening.
 5. An electro-slag furnacecomprising a chamber containing molten slag, means for introducing ametal charge into the molten slag, a first electrode fixed in thechamber, a second displaceable electrode spaced from the first electrodeand constituted by solidified metal derived from said metal charge,means for applying electrical energy to said electrodes, cooling meansfor cooling the chamber between said electrodes for forming a slag crustwhich prevents short circuits between said molten slag and the chamber,said chamber comprising a housing provided with a bottom opening and amold below the opening on extension thereof, said second electrode beingpositioned in said mold, the electrical energy applied to the electrodesforming pools of said metal charge at the surfaces thereof, the heightof said housing disposed between the first and the second electrodesbeing about ten times smaller than the distance between the oppositesides of said opening.